1. Field of the Invention
The present invention relates to a novel olefin polymerization catalyst and a method for polymerizing an olefin using the catalyst. More particularly, the present invention is concerned with an olefin polymerization catalyst comprising a transition metal compound which comprises one transition metal selected from Ti, Zr and Hf, and at least two ligands, wherein one ligand is a group having a cyclopentadienyl skeleton, and the remaining at least one ligand is a monovalent, bidentate, anionic chelating ligand having two coordinating atoms, one of which atoms is selected from O, S, Se and Te, and the other of which atoms is selected from N, S, Se and Te. The present invention is also concerned with a method for efficiently producing an olefin homopolymer or olefin copolymer by using this catalyst.
The transition metal compound of the olefin polymerization catalyst of the present invention is a novel compound. By polymerizing an olefin using the olefin polymerization catalyst of the present invention, there can be advantageously produced an olefin homopolymer having a narrow molecular weight distribution or an olefin copolymer having not only a narrow molecular weight distribution but also a uniform copolymerization distribution (i.e., uniform distribution with respect to the proportions of different component monomer units constituting the copolymer), and the resulting polymer has advantages in that it has high impact strength, high resistance to stress cracking, high transparency, excellent heat sealing properties at low temperatures, high blocking resistance, low tackiness, low extractability and the like.
2. Prior Art
In recent years, olefin polymerization methods have been proposed in which a solvent-soluble transition metal complex is used as a coordination polymerization catalyst. Representative examples of such transition metal complexes can be roughly classified into two groups. One of the two groups of transition metal complexes is a group of the so-called "metallocene compounds", that is, transition metal complexes each comprising a transition metal selected from Group 4A of the Periodic Table and, coordinated thereto, two ligands each having a cyclopentadienyl skeleton. Olefin polymerization methods using a metallocene compound as a catalyst are disclosed in, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 58-19309 (corresponding to EP 069951), 60-35007 (corresponding to EP 129368), 61-130314 (corresponding to EP 0185918), 1-301706 (corresponding to EP 142143) and 2-41303 (corresponding to EP 351392). The other of the two groups of transition metal complexes is a group of the so-called "geometrically constrained compounds", that is, transition metal complexes each comprising a transition metal selected from those metals belonging to Group 4A of the Periodic Table and, coordinated thereto, a crosslinked product between a cyclopentadienyl skeleton-containing molecule and an electron donative molecule, such as amide, wherein the transition metal and the crosslinked product together constitute a transition metal-containing condensed ring. Olefin polymerization methods using a geometrically constrained compound as a catalyst are disclosed in, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 3-163088 (corresponding to EP 416815), 5-194641 (corresponding to EP 502732), 5-230123 (corresponding to EP 514828) and 6-306121. It is known that, with respect to an olefin polymerization method in which any of these transition metal complexes is used as a catalyst and an aluminoxane or a cation generator is used as an auxiliary catalyst, advantages can be obtained in that an olefin homopolymer having a narrow molecular weight distribution can be produced and that a copolymerization can be performed with high copolymerizability, thus enabling the production of an olefin copolymer having uniform copolymerization distribution.
On the other hand, it has also been attempted to use, as an olefin polymerization catalyst, a third-type transition metal complex which contains a transition metal selected from Group 4A of the Periodic Table, but does not belong to any of the above-described two groups of transition metal complexes. However, such a third-type transition metal complex, which does not belong to any of the above-described two groups of transition metal complexes, has low polymerization activity which does not reach the practically acceptable level, as compared to the above-described two groups of transition metal complexes. As an olefin polymerization method using such a third-type transition metal complex, Unexamined Japanese Patent Application Laid-Open Specification No. 5-170820 discloses a polymerization method using, as a catalyst, a transition metal complex comprising a transition metal selected from Group 4A of the Periodic Table and, coordinated thereto, two ligands, namely, a ligand having a cyclopentadienyl skeleton and a chelating ligand having two coordinating oxygen atoms, which complex is represented by the formula CpM(R.sup.1 COCR.sup.2 COR.sup.3).sub.2 X wherein M represents Zr or Hf, Cp represents a group having a cyclopentadienyl skeleton, each of R.sup.1, R.sup.2 and R.sup.3 represents a hydrocarbon group, and X represents a halogen atom or --SO.sub.3 CF.sub.3. As another olefin polymerization method using such a third-type transition metal complex, the Journal of Chemical Society, Chemical Communications [J. Chem. Soc., Chem. Commun., 18, 1415-1417 (1993)] discloses a polymerization method using, as a catalyst, a transition metal complex comprising a transition metal selected from those metals belonging to Group 4A of the Periodic Table and, coordinated thereto, two ligands, namely, a ligand having a cyclopentadienyl skeleton and a chelating ligand having two coordinating nitrogen atoms, which complex is represented by the formula CpM[(NSiMe.sub.3).sub.2 CPh]X.sub.2 wherein Me represents CH.sub.3, Ph represents C.sub.6 H.sub.5, Cp represents C.sub.5 H.sub.5 or C.sub.5 Me.sub.5, X represents a chlorine atom or --CH.sub.2 Ph and M represents Zr, Ti or Hf. The transition metal complex catalysts disclosed in these publications tend to have low activity.
Therefore, it has been desired to develop a new type olefin polymerization catalyst which does not belong to any of the above-described two groups of transition metal complexes, but has high polymerization activity.